A typical automatic transmission includes a hydraulic control system that is employed to provide cooling and lubrication to components within the transmission and to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes arranged with gear sets or in a torque converter. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to various subsystems including lubrication subsystems, cooler subsystems, torque converter clutch control subsystems, and shift actuator subsystems that include actuators that engage the torque transmitting devices. The pressurized hydraulic fluid delivered to the shift actuators is used to engage or disengage the torque transmitting devices in order to obtain different gear ratios.
While previous hydraulic control systems are useful for their intended purpose, the need exists for new and improved hydraulic control system configurations within transmissions which exhibit improved performance, especially during default conditions. A default condition is a hydraulic state that the transmission experiences in the absence of electronic control. A transmission in default no longer has the capability to electronically command solenoids to achieve the desired gear state. The default condition may have been intentionally commanded (e.g. when diagnostics indicate corrupt solenoid drivers, corrupt controllers, controller shutdown at high temperatures) or can occur unintentionally due to a hardware failure (e.g. controller failure, wiring harness failure, solenoid driver failure). For some transmission configurations, the hydraulic control system shifts the transmission to neutral during a default condition rather than a drive or reverse gear ratio which limits the ability to drive the vehicle home during a default.
One solution is to actuate individual clutches using a normally high solenoid that, during a default condition, automatically engage the clutches. However, these solenoids cannot be used in certain transmission configurations and there are certain solenoid types for which a normally high version has not yet been developed. Accordingly, there is a need for an improved, cost-effective hydraulic control system for use in a hydraulically actuated automatic transmission that provides one or more default gear ratios during a default condition.